According to the Illiad, the Trojan war began when Paris of Troy took Helen, the wife of Menelaus the King of Sparta. Paris and Helen went to Troy. Menelaus, the husband of Helen sent armies to help him attack Troy to get Helen back.
As well, Troy was an important city for trade, and the Greeks were looking for any excuse to go to war with it to gain wealth and control.
(c) Mediatech